What is the relay valve ?

A relay valve is an air-operated valve typically used in air brake systems to remotely control the brakes at the rear of a heavy truck or semi-trailer in atractor-trailer combination. Relay valves are necessary in heavy trucks in order to speed-up rear-brake application and release, since air takes longer to travel to the rear of the vehicle than the front of the vehicle, where the front service brakes, foot-valve, parking-control valve, and trailer-supply valve (if applicable) are located.

In a truck’s air brake system, relay valves get a signal when a driver presses the treadle, which then opens the valve and allows air to enter the brake chamber via air inlet. The diaphragm gets pushed, then the rod, then the slack adjuster which twists to turn the brake camshaft. Next, it moves the disc, wedge or s-cam, which pushes the brake shoes and lining, creating friction. This friction slows and eventually stops the brake drum’s turning, which stops the wheel.

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